1. Field of the Invention
This invention relates to a latex binder for cellulosic, fibrous webs which, after application and cure, imparts wet and dry strength to the web. Thereafter the fibers in the web can be recovered for reuse in a subsequent web-making process if desired, because the bonded fibers can be readily unbonded or separated by a repulping process. The fiber recovery (repulping of broke) can be achieved under conditions similar to those normally employed by the paper industry for repulping conventional wet-strengthened webs (e.g. strengthened with wet strength resins) without significant degradation of any of the fiber's useful properties. The binder of the present invention is an aqueous latex containing an addition polymer comprising ethylenically unsaturated amic acid and at least one other ethylenically unsaturated monomer.
2. Prior Art
Conventional latices, such as those employed in a papermaking process disclosed in U.S. Pat. No. 4,158,594, cannot be repulped to recover the fibers in the paper under conventional repulping conditions because the severe repulping conditions needed to chemically attack the latex would destroy the cellulosic fibers. Conventional latex binders for fibrous webs were therefore classified as "non-repulpable" because webs treated with them could be repulped only with extreme difficulty by reason of their strong resistance to hydrolytic depolymerization or solubilization. This is understandable since such latices have evolved from polymers designed for use in water-based latex paints such as house paints where hydrolysis or depolymerization of the paint after drying is meticulously avoided. This resistance to hydrolysis after drying and curing of the latex is achieved by crosslinking of the latex polymers upon curing.
Self-crosslinking latices usually offered in commerce are essentially copolymers and terpolymers of acrylic and methacrylic esters, vinyl esters, olefins, acrylic and methacrylic acids, maleic and fumaric acids, vinylbenzenes, vinylnitriles and N-methylolvinylamides in various combinations and proportions with the N-methylolvinylamide being the source of their self-crosslinking or curing characteristic. An example of such a binder is a latex derived from vinylacetate, ethylene and N-methylolacrylamide. The N-methylolacrylamide (or N-methylolmethacrylamide) contributes a pendant self-crosslinking functionality which does not react until activated by elevated temperature and low pH.
The stability and durability of the crosslinks which form when the latices are cured depends upon the chemical nature of the crosslinks. Methylene crosslinks, as obtained with N-methylol acrylamide monomers incorporated into the latex polymer in conventional latices such as formed when N-methylolvinyl amides (self-crosslinking) or vinyl amides and trimethylolmelamine or vinyl amides and formaldehyde (crosslinkable), are especially inert to alkaline hydrolysis and require a repulping or broking operation which is impracticable or costly.
The nature of such methylene crosslinks is depicted in FIG. 1. ##STR1##
Latices of the nature shown in FIG. 1, usually require excessive amounts of alkali (10-12% sodium hydroxide by weight of fiber) at temperatures from about 200.degree.-220.degree. F. for 2 hours or more in order to effect fiberization (separation of the web into individual fibers). Such severe conditions are detrimental to most natural fibers and to many synthetic fibers resulting in fiber losses of about 15-20%. In addition, degradation products from the cellulosic fibers increase the amount of biodegradable matter in the spent repulping liquor, thereby necessitating costly treatment of the effluent or waste streams.
Recently, it has become desirable to eliminate the use of formaldehyde-based latices as binders for fibrous webs because of the known and suspected toxic effects of formaldehyde. Examples of such formaldehyde-based latices are those which employ N-methylolacrylamide and/or N-methylolmethacrylamide as the source of their self-crosslinking or curing ability. The N-methylol functionality is a latent source of formaldehyde because formaldehyde is released during the curing or crosslinking reaction. The crosslinking reaction between two polymer chains involves the condensation of pendent N-methylol functionalities to form bis-methylene amide crosslinks and formaldehyde.
It is known in the art of making self-crosslinking latices to eliminate the known and potential hazards of formaldehyde-containing latices to make latices whose copolymers contain functional groups capable of reacting with innocuous polyaldehydes such as glyoxal. These systems take advantage of the reaction between amide groups on separate polymer chains and a polyfunctional aldehyde to crosslink the chains. Talet, U.S. Pat. No. 2,886,557, describes the preparation of a crosslinked acrylamide polymer by reaction of a vinyl acetate-acrylamide-containing latex copolymer with glyoxal. Falgiatore et al, in U.S. Pat. No. 4,199,643, describe the preparation of a latex binder which contains a copolymer comprised of ethylenically unsaturated monomers copolymerized with an ethylenically unsaturated vinyl amide--the amide group of which is mono-functionally bonded to a polyaldehyde.
These mono-functionalized aldehyde groups can subsequently react with non-functionalized amide groups to produce a crosslinked polymer after application to the binder to textiles and/or paper. Mackower et al, in Canadian Pat. No. 997,091, describe the preparation and use of cellulose-substantive, glyoxalated latex binders containing polymerizable vinyl amide monomers in their latex copolymers. The reported latices contain copolymers comprised of a vinyl amide and at least one other ethylenically unsaturated monomer. The latex copolymer is at least partially reacted with glyoxal prior to application to the wet end of a paper machine.
A way of avoiding the use of formaldehyde in latex binders which are for application to cellulosic materials is to incorporate into the latex copolymer groups which will react with cellulose thereby affixing the latex copolymer to the cellulosic material.
The general reaction between beta-amic acids and alcohols to form half-acid esters was described by Cuculo in a series of publications in the Textile Research Journal, Volume 41 pages 321-326; Volume 41 pages 375-378; volume 43 pages 283-293; volume 45 pages 314-316; volume 46 pages 393-397. These publications describe the preparation of half-acid esters of cellulose by the facile reaction of beta-amic acids and cellulosic hydroxyl groups.
U.S. Pat. No. 3,555,585 granted to Cuculo on Jan. 19, 1971 describes the preparation of cellulose half-acid esters by reaction of beta-carbamyl or gamma carbamyl acids with cellulose. The disclosure is concerned with modification of non-woven, cellulosic webs to yield disposable fabrics by the formation of cellulose half-acid esters through the reaction of the cellulosic fabric with a carbamyl substituted organic acid.
Kimura, in Japanese Patent Disclosure No. 74 90,390 published Aug. 29, 1974, describes a process for the preparation of acrylic copolymers for use as textile finishing agents. The acrylic copolymers described in this invention are prepared in an aqueous latex and contain acrylic monomers and/or other ethylenically unsaturated monomers and unsaturated amic acids such as maleamic acid. When the latices as described in the invention are used as textile finishing agents, they exhibit outstanding resistance to dry cleaning. The superior dry cleanability of these latices is attributed to the existence of the strong fiber-copolymer bonds formed by the reaction of the amic acid pendent functionalities of the latex copolymer and the cellulose molecules of the cotton fabric. The strong fiber-copolymer bonds are the result of an intramolecular esterification between the latex copolymer and the cellulose of the cotton fabric. These ester linkages are insensitive to the action of dry cleaning solvents such as percholoroethylene. The present inventor has found that the latices described by Kimura for improving dry cleanability of textile fabrics are useful for imparting wet strength to cellulosic webs. Not only does Kimura fail to disclose or suggest such a utility but, moreover, the papermaking art has been in the possession of closely related copolymers without recognizing the value of the ester linkages in providing wet strength to paper webs.
U.S. Pat. No. 3,017,291 granted Jan. 16, 1962 to McLaughlin et al describes the use of emulsion copolymers containing an acid, such as maleic or aconitic, and their salts, as wet strength resins. This patent at Column 1, lines 36-39 contains the naked suggestion that salts of partial amides, such as the salt of maleamic acid, can be produced by neutralization of the emulsion copolymers of the patented invention with ammonia and amines. While there is no subsequent enabling disclosure or teaching in respect of such salts merely named by McLaughlin et al, neutralization, as normally understood by one of ordinary skill in the art and in accordance with the disclosure of an antecedent patent assigned to the same assignee, U.S. Pat. No. 2,999,038, at Column 2, lines 50-54, means that aqueous alkali, particularly ammonia, is added to the copolymer. The resulting compound, in the case hypothesized by McLaughlin et al would contain the diammonium salt of maleic acid, not the salt of a half-acid, half-amide characteristic of a salt of maleamic acid.